In an article by R. Stierlin entitled "Fiber-Optical Sensors" in the Bulletin SEV/VSE [Swiss Electro-Technical Assn.] 82, (1991), 1, pp. 21 to 29, a heptagonal block of quartz glass is used, which has in its interior a circular-cylindrical recess for a current conductor whose current is to be detected. At the edge and vertically to a first lateral face, monochromatic light is introduced into the quartz glass block via a polarizer and, after having been reflected three times at three reflecting lateral faces with respectively one 90.degree. vertical deviation in relation to a second lateral face which, together with the first lateral face, forms a right angle, is directed at the edge via a further polarizer to a photodiode for detecting the light intensity. In accordance with the Faraday effect, the direction of polarization of the linearly polarized light during one passage through the quartz block is rotated proportionally to the magnetic field generated by the current and proportionally to the length of the path traveled through the material of the quartz block. The instantaneous current intensity can be calculated from the value of the angle of rotation. The sensitivity of this current sensor is relatively low.
It is known from "Miniature Faraday Current Sensor Based on Multiple Critical Angle Reflections in a Bulk-Optic Ring" by Y. N. Ning et al. in Optics Letters, vol. 16, No. 24 (1991), pp. 1996 to 1998, to direct light in several circuits inside a circular cylinder around a current conductor in the center and to evaluate it subsequently. A current sensor which focuses in this way operates close to the angle of total reflection; it is hard to adjust. Applied dielectric coatings pose a problem because of possible contamination.
From "Accurate Faraday Effect Current Sensor" by Toshihiko T. Yoshito in Advances in Optical Fiber Sensors, 1991, pp. 208 to 217, it is known to employ pairs of coats of SiO.sub.2 /TiO.sub.2 and MgF.sub.2 /ZnS for coating reflector surfaces in order to reduce the effects of phase shifting during reflection.